TITUSVILLE — A circuit judge has asked for an investigation into whether a Palm Beach dog trainer committed perjury when he testified that controversial dog handler John Preston forged a training certificate from his school.

Documents turned over to the state attorney's office Wednesday by Circuit Judge Dean Moxley, who as a prosecutor used Preston's tracking dogs in several cases, show Palm Beach dog trainer Tom McGinn testified in 1977 and 1984 that Preston attended his school.

The furor over Preston dates to 1984, when he failed a court-ordered tracking test after he appeared as an expert witness who helped convict about 15 defendants in Brevard County. One man is on death row.

The state attorney's office began investigating Preston last fall after McGinn said on ABC's 20/20 that a training certificate Preston said he received in 1975 was a forgery.

Contacted at his Jupiter home, McGinn still contends the certificate is forged and said he fears prosecutors started the investigation to make him a scapegoat.

''They have been taken in by Preston and they don't want to admit it,'' he said.

The latest accusations came a week before McGinn is scheduled to testify against Preston at a hearing for Nick Lennear, convicted of burglary and battery in 1983.

Preston testified his dog tracked Lennear's scent to the scene of the crime.

Lennear's attorney, Brian Onek, claims his 24-year-old client deserves a new trial because Preston might have lied on the stand when he said he received 240 hours of training at McGinn's school.

Onek said Wednesday he will proceed with the hearing set for May 1 before Orlando Circuit Judge Ted Coleman and will call McGinn to testify.

''Someone is lying, but I don't know who,'' he said. ''I'm going to let the judge decide who's committing perjury.''

Florida Bar rules prohibit attorneys from knowingly using perjured testimony in a courtroom and those who do could be sub-

ject to disciplinary actions.

Judge Moxley prosecuted Lennear and also will testify at that hearing, which was moved to Orlando after Circuit Judge John Antoon excused himself from the case because he is a friend of Moxley's.

Moxley declined to comment on his request for the investigation but said in a letter to prosecutors that ''having become aware of this alleged violation . . . I feel it is incumbent upon me to relate to you this complaint.''

The new documents will not end prosecutors' investigation of Preston, Assistant State Attorney Wayne Holmes said. Those findings will be released after the May 1 hearing.